Preliminary estimates of the physiological rates of methylneogenesis, trans- methylation, and methionine thiomethyl recycling in Lemna indicate that these processes are not regulated by methionine or one of its products. The following evidence has been obtained to support our previous proposal that methionine thiomethyl recycling proceeds via conversion of methylthioadenosine to methionine: (i) The growth of Lemna deprived of methionine by administration of lysine + threonine and/or propargylglycine is restored by supplementation with methylthioadenosine. (ii) Incubation of Lemna with methylthioadenosine labeled in the methyl and adenosyl moieties indicate that the methylthio and 4-carbon moieties of methionine were derived, respectively, from the methylthio and four ribose carbons, of methylthioadenosine. (iii) Cell-free extracts of Lemna convert methylthioadenosine to methionine, with methylthioribose-1-phosphate tenatively identified as an intermediate. A sensitive and specific assay was developed for threonine synthase, and optimal conditions established for its assay, extraction and stability. The enzyme from Lemna resembles that described from a variety of plants by other workers in being markedly stimulated by S-adenosylmethionine.